Conventionally, there is water jet peening for reforming a tensile residual stress generated in a welded portion to a compressive residual stress. The water jet peening jets, under water, high-pressure water to create cavitation bubbles caused by cavitation toward a surface of the welded portion to provide the surface of the welded portion with an impact pressure due to collapse of the cavitation bubbles, thereby to reform the tensile residual stress on the surface of the welded portion to the compressive residual stress. A device (water jet peening device) that performs such water jet peening is provided with a jet nozzle for jetting the high-pressure water. Water jet peening devices provided with a jet nozzle are described in Patent Literatures 1 to 5 below.